Wisconsin

Researchers trace genetic origins of electric organ in fish

The South American electric eel and hundreds of other electric fish evolved in six distinct lines over a period of 400 million years, developing an electric organ from what had once been muscle and providing a fascinating laboratory for the study of evolution.

Now, in a paper published in the journal Science, researchers from University of Wisconsin-Madison and more than half a dozen other institutions have used genome sequencing and other techniques to trace the genetic and molecular underpinnings of the electric organ, a trait that fascinated Charles Darwin.

"They are the only organisms on earth that kill with electricity. Lots of organisms use chemicals and other methods," said Michael Sussman, one of the authors of the paper in Science and director of the UW Biotechnology Center. The electric eel produces a charge of about 100 volts for every foot of length — as much as 600 to 800 volts in total.

The new study produced a draft sequence of the South American electric eel's genome and also revealed some 30 genes that have formed a kind of standard tool kit for the development of an electric organ in different species of fish.

The electric organ is interesting from an evolutionary perspective because it evolved independently six different times. Fish use the feature to capture prey and defend themselves. They also use the electric organ to navigate through the murky waters of the Amazon. By throwing out electric discharges and measuring how quickly the signals bounce back, an electric eel can form a picture of rocks and other features that it cannot see.

Fish and eels also use the electric organ to communicate with a potential mate, the equivalent of a bird's mating song.

Understanding how nature produced this unusual characteristic raises intriguing, futuristic possibilities. If scientists learn how a muscle became an electric organ perhaps they will one day be able to engineer a pacemaker from our own native muscle, one that would not require battery changes.

"We could almost evolve an electric organ in the lab," said Jason R. Gallant, co-lead author of the study and an assistant professor of zoology at Michigan State University. The other lead author was Lindsay L. Traeger, a graduate student in genetics at UW-Madison, who was assisted by another UW-Madison graduate student, Jeremy Volkening.

Gallant helped the team arrive at common genetic characteristics of an electric organ by comparing genes in the electric eel with genes in two other electric fish, the African electric catfish and the elephant fish.

Changes like the one that saw a muscle evolve into an electric organ occur through mutations, or anomalies in the genetic code.

The change was likely to come in water because it is a good conductor of electricity, unlike air. The theory goes that fish had been using muscles to produce bubbles in the water in an effort to attract a mate. Bubbles, however, also turned out to be a tip off to predators which then ate the fish. Sending electric signals through the water offered the advantage of stealth — attracting the mate, but not the predator.